Gemara

(Rav Papi): If Reuven made a Shali'ach to take Ma'aser Behemah, and the Shali'ach called the ninth "Asiri," it is Kodesh. If he called the 11th "Asiri," it is not Kodesh.

(Rav Papa): Also the ninth is not Kodesh, for Reuven can say 'I made you a Shali'ach to benefit me, but not to harm me!'

Question (Mishnah): Reuven's Shali'ach to take Terumah should take the amount that Reuven wants;

If he does not know how much Reuven wants to take, he should take one part in 50, like average people do. If he took one in 40 or one in 60, it is Terumah.

Answer: Regarding Terumah, some (i.e. generous) people normally take one in 40. The Shali'ach can say 'I thought that this is what you wanted';

Here, the Shali'ach erred.

Bava Basra 169b: A case occurred in which Leah gave money to Levi to buy land for her. He bought for her without Acharayus (liability to compensate the buyer if it is legally taken from her).

Rav Nachman: She sent you to help her, and not to harm her! You must buy the land without Acharayus, and sell it to her with Acharayus.

Kidushin 42b (Rav Nachman): If brothers divided an inheritance and erred, i.e. some received too much and others too little, the law is like that of buyers. If the mistake was less than a sixth of the value, the sale (or division) stands;

(Rava): This is when they themselves divided. If a Shali'ach divided for them, it is invalid. They can say 'we appointed you to help us, not to harm us!'

Gitin 65b (Beraisa #1): If Reuven told a Shali'ach 'place an Eruv of dates for me', and he placed an Eruv of figs, the Eruv is valid;

Contradiction (Beraisa #2): The Eruv is Pasul.

Answer #1 (Rabah): Beraisa #1 is like R. Eliezer, who holds that a Meshale'ach merely suggests the easiest way to fulfill the mission, but he is not insistent that it be done that way. Beraisa #2 is like Chachamim, who hold that he is insistent that the Shali'ach do like he said.

Answer #2 (Rav Yosef): Both Beraisos are like Chachamim. One discusses an Eruv from Reuven's dates, and the other discusses an Eruv from another's dates.

Rishonim

Rambam (Hilchos Sheluchim 1:3): If one gave money to a Shali'ach in order to buy land, and he bought it without Acharayus, he blundered. The Shali'ach buys it for himself without Acharayus, like he did, and sells it to the Meshale'ach with Acharayus, since he bought it with his own money. Therefore, if he stipulated to make him a Shali'ach whether to help or blunder, even if he sold property worth 100 for 1, or bought property worth 1 for 100, he cannot retract. The Meshale'ach must pay according to the condition.

Ra'avad: Indeed, this is like the Gemara. However, I have a question. Since a mistake of any amount ruins the Shelichus and voids the sale, why did Rav Nachman obligate the Shali'ach to buy it for himself without Acharayus, and sell it to her with Acharayus? The sale should be Batel, since he erred in the Shelichus! It seems that any deviation nullifies Shelichus only when the Meshale'ach wants to annul it. That Meshale'ach wanted the sale, therefore Rav Nachman obligated the Shali'ach to fix his blunder. The same applies to all Shelichus. The Rambam distinguished between whether or not the Shali'ach informed the seller that he is a Shali'ach. He can say that this was the case with Rav Nachman.

Rosh (Bava Basra 10:26): Seemingly, Levi (the Shali'ach) had told the seller that he was buying a field for Leah. The Gemara said 'he went and bought for her.' Also, Rav Nachman told Levi 'go buy from him without Acharayus...' If Levi had not said that he buys for Leah, why did he need to buy again? He bought Stam, for himself! Rather, he had told the seller. He deviated from the Shelichus, for normally, one who buys a field buys with Acharayus, for people do not throw away their money. Therefore the Shelichus was Batel, and the sale is void. Therefore, Levi needed to buy it again. However, if Levi wants, he can return Leah's money to her. Rav Nachman taught that if he does not have the money, he must accept Acharayus.

Mordechai (Kidushin 508): Some explain that the buyer and seller knew at the time of the sale that he is a Shali'ach, but if he did not know, and he erred less than a sixth, the sale stands, and the Shali'ach who blundered pays and loses. Rabbeinu Yehudah ben Klonimus explains that even if he did not know that he is a Shali'ach, and afterwards he found out through witnesses, the sale is void. If it is not clarified, the sale stands and the Shali'ach pays, like the case in Bava Basra. Some reject this. Rather, since some people would not refrain from buying due to concern for Acharayus, the sale was valid. An error in (the value of) the sale item is different. (That invalidates the sale.)

Machaneh Efrayim (Shutfim 2): The Rambam says that if a Shali'ach did not inform the seller that he is a Shali'ach, the sale stands. The Rosh says that the Shali'ach bought it for himself. It seems that this is only if he was Mezid, but if he was Shogeg and made a common mistake, he does not lose. If there are witnesses that he was a Shali'ach, the sale is Batel. If not, the sale stands. The Mordechai learns like this. Some say that the Rambam holds that whenever he did not say that he is a Shali'ach, he suffers any loss due to his mistakes.

Rashi (61a DH Kara): Calling the ninth "Asiri" is not a major loss. It will be eaten after it gets a Mum. If he called the 11th "Asiri" if it would become a Shelamim, the Chazah v'Shok would be given to Kohanim, which is a loss for the owner. Rav Papa holds that also making the ninth Kodesh is a loss, for he must wait for it to get a Mum, and he cannot shear it or work with it.

Rashi (65b DH Ha): In Beraisa #1, Reuven asked to make an Eruv from Reuven's dates. One does not care which food is used for his Eruv. In Beraisa #2, Reuven asked to make an Eruv from Shimon's dates. Reuven had permission only for this, but not to take Shimon's figs. Therefore, the Eruv is Pasul.

Tosfos (65b DH Ha): In Beraisa #1, Reuven asked to make an Eruv from the Shali'ach's dates. Reuven does not care if the Shali'ach used his own figs instead. In Beraisa #2, Reuven asked to make the Eruv with Reuven's dates. We say that he wanted specifically the dates.

Poskim

Shulchan Aruch (CM 182:2): If the deviated from the owner's intent, he did nothing. This is only if he said 'I am Ploni's Shali'ach.' therefore, even if he bought or sold through Meshichah, if it is found that he transgressed Ploni's intent, the sale is void.

Gra (7): See Gitin 29a (if Reuven said 'take my object from her when you give the Get...' the Shali'ach may not make another Shali'ach. Perhaps Reuven is adamant to get the object, and he does not rely on the other Shali'ach. R. Yochanan taught that if he made another Shali'ach, the Get is Batel. It is as if he commanded 'divorce her only in the house', and the Shali'ach divorced her in the attic, or 'divorce her only using the right hand', and he used the left hand). Also see 65a (if a man told a Shali'ach 'give this Get to my wife in the place Ploni, and the Shali'ach gave it elsewhere, the Get is Pasul).

Shulchan Aruch (ibid.): If he did not say that he is Ploni's Shali'ach, the sale stands, and the case is between him and Ploni.

Chachmas Shlomo (Kesuvos 99b DH d'Ta'ah): It seems that even if the buyer did not know that the seller is only a Shali'ach, the sale is void, for one cannot sell what is not his. However, if a Shali'ach deviated and bought, even though the sale is void, if the seller did not know that he is a Shali'ach, the seller keeps the money. The Shali'ach must pay, for he deviated. The seller need not return the money, like we find in Bava Basra.

Nesivos ha'Mishpat (Bi'urim 4): Even if he was a Shali'ach to sell, and it became clear that he sold the Meshale'ach's property and transgressed his words, even so the sale stands. Even though one cannot sell what is not his, here is different, for surely the Shali'ach intended that the sale stand and he will fix his blunder. A proof is that if he was a Shali'ach to buy and deviated, and the Shali'ach wants to keep the item for himself and return the Meshale'ach's money, he cannot. He must give it to the Meshale'ach and fix his blunder (Sa'if 6). This shows that Chazal estimated the Shali'ach's intent that the sale stand and he will fix his blunder, lest he be like a thief.

Nesivos ha'Mishpat: If the Shali'ach cannot fix his blunder, seemingly he could void the sale through showing that he transgressed the Shelichus, since the Meshale'ach consented only to what he told the Shali'ach. Since the Shali'ach cannot fix matters, he is like one who sold what is not his. Likewise, if he bought and deviated, this is like one who was Mekadesh with a stolen object; she is not Mekudeshes. We should say that he takes the money back from the seller, and the sale is Batel if there was no other Kinyan. If there was another Kinyan, e.g. Meshichah, the sale stands between the seller and the Shali'ach. The seller can force him to pay other money; the Shali'ach must return the Meshale'ach's money to the Meshale'ach. However, if the Shali'ach can pay, even if the only Kinyan was Kesef, the sale stands, even if he can show that he deviated and the money was not his. However, in Siman 185:5 we find that the sale stands whenever he did not inform (that he is a Shali'ach). This is unlike the Chachmas Shlomo.

Pischei Teshuvah (5): Be'er Heitev (6) says that it does not help if he stipulated 'whether to help (or blunder).' Sha'ar Mishpat (3) was astounded at this, for it is unlike 123:5. See what I wrote there.

Shulchan Aruch (123:5): If one has Harsha'ah (power of attorney) and he pardoned the other party or compromised with him, it is invalid, for the owner says 'I sent you to benefit me, but not to harm me!' If he stipulated 'to help or harm me', even if he pardoned everything, it is pardoned.

Pischei Teshuvah (2, citing Sha'ar Mishpat 5): Some say that if the lender tells the borrower 'I did not authorize him to compromise', this is like a Shali'ach who erred. If the Meshale'ach cannot prove that he was a Shali'ach Stam, the seller can say 'you stipulated 'to help or harm me'', so ha'Motzi mi'Chavero Alav ha'Re'ayah (he need not return the money). Also here, the borrower can say 'I do not believe that he deviated.' Others distinguish. The seller does not know if he had any obligation (to return the Meshale'ach's money), therefore he is exempt. The borrower does not know whether he paid his debt, so he is liable. I say that if the Shali'ach said 'the lender commanded me to collect this amount and pardon the rest', he is believed, like a third party.